Release of K+ and H+ from poly U in aqueous solution upon gamma and electron irradiation. Rate of strand break formation in poly U

Effect of sodium and acetate ions on 8-hydroxyguanine formation in irradiated aqueous solutions of DNA and 2'-deoxyguanosine 5'-monophosphate

PURPOSE

The aim of this work was to study the combined effect of sodium and acetate ions on the radiation yield of 8-hydroxyguanine (8-OHG), one of the major DNA base lesions induced by free radicals.

MATERIALS AND METHODS

Aqueous solutions of DNA and 2'-deoxyguanosine 5'-monophosphate (dGMP) with various concentrations of sodium acetate and sodium perchlorate were γ-irradiated, enzymatically digested and analyzed by high-performance liquid chromatography (HPLC) methods.

RESULTS

It was found that both salts decrease the 8-OHG radiation yield in the concentration range studied for both DNA and dGMP, except in the case of dGMP wherein an increase in yield occurs in the concentration range from 0.1-1 mM. The dependence of the 8-hydroxy-2'-deoxyguanosine radiation yield on the concentration of both sodium acetate and sodium perchlorate have different shapes and have steeper slopes for the DNA compared with the dGMP solutions.

CONCLUSIONS

The observed decrease in the radiation yield of 8-OHG with increasing concentrations of sodium acetate is consistent with the hypothesis that sodium acetate produces two concentration-dependent effects in the DNA solutions: (1) A conformational change in the DNA caused by Na(+) counterions; and (2) free radical reactions related to the radiolysis of acetate ion.

Photooxidation of nucleic acids on metal oxides: physico-chemical and astrobiological perspectives

Photocatalytic oxidation of nucleic acid components on aqueous metal oxides (TiO(2), α-FeOOH, and α-Fe(2)O(3)) has been studied. The oxidation of purine nucleotides results in the formation of the purine radical cations and sugar-phosphate radicals, whereas the oxidation of pyrimidine nucleotides other than thymine results in the oxidation of only the sugar-phosphate. The oxidation of the thymine (and to a far less extent for the 5-methylcytosine) derivatives results in deprotonation from the methyl group of the base. Some single stranded (ss) oligoribonucleotides and wild-type ss RNA were oxidized at purine sites. In contrast, double stranded (ds) oligoribonucleotides and DNA were not oxidized. These results account for observations suggesting that cellular ds DNA is not damaged by exposure to photoirradiated TiO(2) nanoparticles inserted into the cell, whereas ss RNA is extensively damaged. The astrobiological import of our observations is that the rapid degradation of monomer nucleotides make them poor targets as biosignatures, whereas duplex DNA is a better target as it is resilient to oxidative diagenesis. Another import of our studies is that ds DNA (as opposed to ss RNA) appears to be optimized to withstand oxidative stress both due to the advantageous polymer morphology and the subtle details of its radical chemistry. This peculiarity may account for the preference for DNA over RNA as a "molecule of life" provided that metal oxides served as the template for synthesis of polynucleotides, as suggested by Orgel and others.

Reaction of Dithiothreitol and Para-nitroacetophenone with Different Radical Precursors of .OH Radical-induced Strand Break Formation of Single-stranded DNA in Anoxic Aqueous Solution

The yields of single-strand breakage (ssb) in single-stranded calf thymus DNA (ssDNA) have been determined after 60Co gamma-irradiation of aqueous anoxic solutions in the presence of different concentrations of dithiothreitol (DTT), ascorbate or trans-4,5-dihydroxy-1,2-dithiane, using low-angle laser light scattering. The influence of DTT on the kinetics of ssb formation has been determined by conductivity measurements in pulse radiolysis. The results suggest that strand breakage in ssDNA proceeds via two modes of about equal contribution and with half-lives of about 7 ms and 0.8s, respectively. Both modes reflect reactions of at least two DNA radicals, which react with DTT by hydrogen-atom transfer reactions with similar rate constants of about 5-9 x 10(5) dm3 mol-1 s-1. These hydrogen-atom transfer reactions inhibit strand break formation. The slow mode is shown to represent the decay of base-radicals to generate sugar radicals. The involvement of the oxidizing .OH adduct radical of guanine in the formation of strand breaks can be ruled out and there is no evidence for a contribution from the anion or radical anion of DTT to the inhibition of strand breaks via electron transfer reactions to DNA radicals.

Formation of sugar radicals in RNA model systems and oligomers via excitation of guanine cation radical

In previous work, we have shown that photoexcitation of guanine cation radical (G*+) in frozen aqueous solutions of DNA and its model compounds at 143 K results in the formation of neutral sugar radicals with substantial yield. In this report, we present electron spin resonance (ESR) and theoretical (DFT) evidence regarding the formation of sugar radicals after photoexcitation of guanine cation radical (G*+) in frozen aqueous solutions of one-electron-oxidized RNA model compounds (nucleosides, nucleotides and oligomers) at 143 K. Specific sugar radicals C5'*, C3'* and C1'* were identified employing derivatives of Guo deuterated at specific sites in the sugar moiety, namely, C1'-, C2'-, C3'- and C5'-. These results suggest C2'* is not formed upon photoexcitation of G*+ in one-electron-oxidized Guo and deuterated Guo derivatives. Phosphate substitution at C5'- (i.e., in 5-GMP) hinders formation of C5'* via photoexcitation at 143 K but not at 77 K. For the RNA-oligomers studied, we observe on photoexcitation of oligomer-G*+ the formation of mainly C1'* and an unidentified radical with a ca. 28 G doublet. The hyperfine coupling constants of each of the possible sugar radicals were calculated employing the DFT B3LYP/6-31G* approach for comparison to experiment. This work shows that formation of specific neutral sugar radicals occurs via photoexcitation of guanine cation radical (G*+) in RNA systems but not by photoexcitation of its N1 deprotonated species (G(-H)*). Thus, our mechanism regarding neutral sugar formation via photoexcitation of base cation radicals in DNA appears to be valid for RNA systems as well.

Inhibition of OH radical-induced strand break formation of poly(U) by Ru(bpy)32+ or Ru(phen)32+ attached to the polynucleotide

Reactions of OH radicals with poly(U) (polyuridylic acid) in the presence of Ru(bpy)32+ or Ru(phen)32+ in aqueous solutions were studied. OH radicals were produced by pulse radiolysis and their reactions with ruthenium complexes were measured spectrophotometrically under conditions were the complexes are attached to the polynucleotide. The OH radical adds to either the uracil moiety or the ruthenium complexes. The ratio of the radicals produced depends only on the ratio of their rate constants and the concentrations of poly(U) and ruthenium complexes. Similar results were obtained with uridine-5'-monosphosphate, where the ruthenium complexes are not attached to the nucleotide. Surprisingly, the yield of single-strand break formation from the OH adducts of uracil in poly(U) is much smaller than that expected on the basis of the yield measured in the absence of ruthenium complexes. Possible reasons for this behaviour are discussed.

Influence of nucleic acid base composition on radiation-induced strand breakage in single stranded DNA: a time resolved study

The following study investigates the pathways involved in the induction of single strand breaks (ssb) in various samples of single stranded (ss) DNA (calf thymus, Micrococcus lysodeikticus, Clostridium perfringens) with differing nucleic acid base composition. The time scale for the induction of ssb was determined from changes in the light scattering intensity following pulse irradiation of aqueous solutions containing these ssDNA samples at pH7.8 under either aerated or deaerated conditions. The induction of ssb under these conditions is predominantly by the hydroxyl radical and shows various kinetically distinct components. The immediate ssb (t < 0.02 s) account for approximately 40-60% of the total yield of ssb. The majority of these ssb are suggested to arise from the 'common' initial attack of the hydroxyl radicals at the sugar phosphate backbone for each of the three DNA samples. Furthermore, slower components for ssb formation (t > 0.02 s) were observed and are suggested to occur through base radical mediated H-atom abstraction from the sugar moiety. The half lives for formation of the majority of ssb, formed through this base radical-mediated H-atom abstraction(s), are in the range of 20-43 ms. The yields of these 'base-mediated' ssb vary markedly (under both aerobic and anaerobic conditions) and reflect the base composition of the DNA sample. It is suggested from these studies that the OH-induced base radicals of guanine/cytosine are more effective precursors for strand breakage than those from adenine/thymine in ssDNA.

The role of hydration and radiation quality in the induction of DNA damage--chemical aspects

The mutagenic and lethal effects of ionising radiation are thought to result from chemical modifications induced within DNA. This DNA damage is significantly influenced by the chemical environment and the radiation quality (LET). Water closely associated with the DNA and its immediate environment is involved in the early chemical pathways which lead to the induction of DNA damage and is reflected in the cellular radiosensitivity. For instance, hydration of DNA influences hole migration leading to its localisation at guanine. Changes in the radiation quality are discussed in terms of the complexity of the radical clusters produced. It is inferred that at higher LET, the influence of the chemical environment (O2 etc) decreases with respect to DNA damage and cellular radiosensitivity. It is therefore important to include these effects of environment of the DNA upon the early chemical pathways in models of radiation action.

Aminothiols linked to quinoline and acridine chromophores efficiently decrease 7,8-dihydro-8-oxo-2'-deoxyguanosine formation in gamma-irradiated DNA

In a search for more active radioprotective compounds, we have prepared and examined a series of model molecules in which the radioprotective beta-aminothiol unit (free or derivatized as acetate or phosphorothioate) is tethered to the DNA-binding chromophores quinoline and acridine through links of variable length. The modifying activity of these 'hybrid' molecules was estimated by measuring the formation of 8-oxo-2'-deoxyguanosine (8-oxodGuo) in double-strand DNA upon exposure to gamma-rays in oxygen-free solution in the presence of the drugs. We show that all hybrid molecules protect the guanine moiety from oxidation more efficiently than the parent beta-aminothiol units. The degree of protection is the highest for the molecules in which the thiol is linked to the strong binding intercalator acridine through a long polyaminochain.

Induction of strand breaks in polyribonucleotides and DNA by the sulphate radical anion: role of electron loss centres as precursors of strand breakage

The interaction of the sulphate radical anion, SO4.-, with the polyribonucleotides, poly U and poly C, in deaerated, aqueous solutions at pH 7.5 results in strand breakage (sb) with efficiencies of 57 and 23%, respectively, determined by time resolved laser light scattering (TRLS). Most sb are produced within 70 microseconds, the risetime of the detection system. Oxygen inhibits the induction of sb in poly U and poly C by SO4.- through its interaction with a radical precursor to sb. In contrast, the interaction of SO4.- with poly A and single stranded DNA does not lead to significant strand breakage (< or = 5% efficiency). From optical studies, the interaction of poly A and poly G with SO4.- radicals yields predominantly the corresponding one electron oxidized base radicals. With poly C and poly U, it is proposed that the SO4.- radical interacts predominantly by addition to the base moiety to produce the C(5)-yl and C(6)-yl sulphate radical adducts which react with oxygen. These base adducts subsequently interact with the sugar-phosphate moiety by H-atom abstraction to yield C(2)' sugar radicals with rate constants in the range 1.3-1.7 x 10(5) s-1. It is proposed that the C(2)' sugar radical leads to strand breakage within 70 microseconds, in competition with its transformation into the C(1)'-sugar radical involving base release. From optical studies on the interaction of SO4.- with double stranded DNA, it is suggested that the predominant radical species produced in DNA is the one-electron oxidized radical of guanine, consistent with positive charge migration in DNA. Since the efficiency of SO4.- to induce sb in single stranded DNA is low, it is concluded that the one-electron oxidized guanine radical does not effectively induce strand breakage in DNA.

Photoinduced interaction of Ru(bpy)3 2+ with nucleotides and nucleic acids in the presence of S2O8 2-; a transient conductivity study

The photochemical reactions of Ru(bpy)3(2+) with single- and double-stranded DNA, polynucleotides and purine-containing nucleotides in argon-saturated aqueous solution in the presence of S2O8(2-) were studied using time-resolved absorption and conductivity methods. The conversion of Ru(bpy(3(3+) to Ru(bpy)3(2+), monitored spectroscopically either after rapid mixing with substrate or after laser flash excitation (lambda exc = 353 nm) is quantitative at nucleotide-to-sensitizer ratios [N]/[S] of 1-2 for DNA and other guanine-containing compounds. Conductivity measurements following the laser pulse revealed a fast conductivity increase (rise time, less than 0.1 ms) due to the formation of protons and, to a lesser degree, to charged species of much lower ion mobility. A slower component in the 0.01-1 s range was observed for nucleic acids; its amplitude is markedly reduced at pH 6-9. In buffered neutral solution the signal is replaced by a slight decrease in conductivity. Electronically excited Ru(bpy)3(2+) bound to DNA reacts with S2O8(2-) to form Ru(bpy)3(3+) and SO4(.-) as primary oxidizing species both of which react with bases. The resulting base radicals react subsequently with Ru(bpy)3(3+) and Ru(bpy)3(2+) or the ligands in the ruthenium complex, producing protons which give rise to the slower conductivity increase. The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow component. The transient conductivity behaviour is sensitive to the single- or double-stranded nature of DNA.

Kinetics of radiation-induced strand break formation in single-stranded pyrimidine polynucleotides in the presence and absence of oxygen; a time-resolved light-scattering study

Time-resolved reductions in the light-scattering intensity (LSI) of aqueous oxic and anoxic solutions of poly-C and poly-U at pH 7.8, following pulse-irradiation, have been studied as indices of strand break formation. With doses of 3-24 Gy per pulse, a number of kinetically distinct strand breakage components have been detected. A comparison of the LSI responses obtained from irradiations conducted under N2O with those conducted under air or O2 show no marked difference in the overall extent of LSI change. However, the immediate and fast (t 1/2 less than or equal to 50 microseconds) reduction in LSI, accounting for about 18-19% of the pyrimidine polynucleotide's total LSI response in oxic solution, is reduced in the absence of oxygen, to about 12% of the total LSI response found with poly-C and to about 9% for poly-U. For poly-C there is a five-fold enhancement in the rate of major strand breakage under anoxia [k1(N2O) = 7.9s-1] whereas for poly-U a more modest enhancement (about two-fold) is observed. These enhanced rates are mirrored by those for the losses of the principal optical anoxic absorptions (observed pulse radiolytically) that are assigned to the pyrimidine 6-yl base radicals. Such findings support a proposal that the rate-limiting step of major strand breakage for pyrimidine polynucleotides is a base radical mediated hydrogen atom abstraction reaction (Lemaire et al. 1987, Hildenbrand and Schulte-Frohlinde 1989). Irradiation of poly-C and poly-U in N2O/O2 (4:1, v/v) saturated solutions yields LSI changes much larger than those noted under N2O and air (or O2), which are in turn approximately double the responses observed under N2. This indicates that the major strand breaking species of water radiolysis is the OH-radical and that there is an oxygen enhancement of single strand breakage of about 1.9 for poly-C and 1.6 for poly-U.

Rates for repair of pBR 322 DNA radicals by thiols as measured by the gas explosion technique: evidence that counter-ion condensation and co-ion depletion are significant at physiological ionic strength

Rates of repair of pBR 322 plasmid DNA radicals by thiols of varying net charge (Z) at pH 7 and physiological ionic strength were measured using the oxygen explosion technique. The extent of conversion of supercoiled to relaxed circular plasmid was measured by HPLC as a function of the time of oxygen exposure before or after irradiation, the time-courses being fitted by a pseudo-first-order kinetic expression with k1 = k2[RSH]. Values of k2 (M-1 S-1) were: 2.1 x 10(5) (GSH, Z = -1), 1.4 x 10(6) (2-mercaptoethanol, Z = 0), 1.2 x 10(7) (cysteamine, Z = +1), 6.6 x 10(7) (WR-1065 or N-(2-mercaptoethyl)-1,3-diaminopropane, Z = +2). The approximately 6-fold increase in rate with each unit increase in Z is attributed to concentration of cationic thiols near DNA as a consequence of counter-ion condensation and reduced levels of anionic thiols near DNA owing to co-ion depletion. The results are quantitatively consistent with chemical repair as a significant mechanism for radioprotection of cells by neutral and cationic thiols under aerobic conditions, but indicate that repair by GSH will compete effectively with oxygen only at low oxygen tension.

The effects of counter-ion condensation and co-ion depletion upon the rates of chemical repair of poly(U) radicals by thiols

Bimolecular rate constants for reactions of poly(U) radicals with a series of thiols of varying net charge (Z) were measured by pulse radiolysis with conductivity detection at low ionic strength. At pH 7 and 18 degrees C the values of k2 (M-1s-1) were: reduced glutathione (Z = -1), less than 500; 2-mercaptoethanesulphonic acid (Z = -1), 1.5 x 10(3); 2-mercaptoethanol (Z = 0), 1.8 x 10(5); cysteine (Z = 0), 2.0 x 10(5); cysteamine (Z = +1), 4.1 x 10(7). Values determined at pH 4 were: 2-mercaptoethanol, 6.1 x 10(5); cysteamine 2.2 x 10(8); N-(2-mercaptoethyl)-1,3-diaminopropane (WR-1065, Z = +2), 4.6 x 10(8). The variation in rate with structure could not reasonably be attributed to inherent reactivity differences in the thiols and was ascribed to inhomogeneous distributions of the thiols in solution resulting from electrostatic interactions. Thus, cationic thiols are concentrated approximately 100-fold near poly(U), relative to neutral thiols, as a consequence of counter-ion condensation, whereas anionic thiols have approximately 100-fold lower concentration near poly(U) than neutral thiols as a result of co-ion depletion. These results show that the ability of a thiol to repair radical sites in a polyanion is dramatically influenced by its net charge as a consequence of the counter-ion condensation and co-ion depletion phenomena.

The chemistry of free-radical-mediated DNA damage

In the living cell, ionizing radiation can cause DNA damage by the direct effect (ionization of DNA) and the indirect effect (reaction of radicals formed in the neighborhood of DNA with DNA, e.g., OH, eaq-, H, protein- and glutathione-derived radicals). Properties of the base radical cations have been studied in model systems using SO4- radical to oxidize the nucleobases in aqueous solution. The pKa values of some nucleobase radical cations are reported, so are the ensuing reactions of the thymidine radical cation with water. The products of reactions are compared with those formed by OH radical attack. The reaction of eaq- with the nucleobases yields radical anions. Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported. A brief account is given on the reaction of carbon-centered radicals with the nucleobases. These reactions may mimic the reactions of protein-derived radicals with DNA. Glutathione is present in cells at rather high concentrations and is expected to act as an H- or electron-donor in repairing radiation-induced DNA damage (chemical repair). As thiyl radicals are known to also undergo the reverse reaction, i.e., H-abstraction from suitable solutes, some experiments are reported which probe this type of reaction with dilute DNA solutions. In some polynucleotides radical transfer from the base radical to the sugar moiety occurs with the consequence of strand breakage and base release. Some currently held mechanistic concepts are discussed. Attention is drawn to some important open questions which should be addressed in the near future.

Single- and double-strand break formation in double-stranded DNA upon nanosecond laser-induced photoionization

Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA.

The kinetics of radiation-induced strand breakage in polynucleotides in the presence of oxygen: a time-resolved light-scattering study

The time-resolved light-scattering changes of aqueous, aerated solutions of poly-C, poly-U and poly-A at pH 7.8, following pulse irradiation, have been studied as indices of strand break formation. With doses of 4-24 Gy/pulse a number of kinetically distinct components have been detected. For the poly-pyrimidines an immediate and fast reduction (tau 1/2 less than or equal to 50 microseconds) in light-scattering intensity (LSI), accounting for approximately 20% of the total LSI change, is followed by a much slower loss (k1 approximately 1.6 s-1) which constitutes their major LSI change. For poly-A a similar fast component is observed, present to an extent equivalent to the one noted with poly-C; it constitutes, however, over 50% of the purine polynucleotide's total response, with the remainder of the change being a slower loss (tau 1/2 approximately 0.09 s). Optical pulse radiolysis studies of poly-C and poly-U, in support of the LSI investigations, show that transient absorbances in a region assigned to base peroxyl radicals decay in a complex fashion, with some at a rate equivalent to that for the slow (major) component of LSI loss. These observations support a proposal that the rate-limiting step of major strand breakage for these polynucleotides, in the presence of oxygen, is a base peroxyl radical-mediated abstraction of a H-atom from an adjacent sugar moiety (Bothe et al. 1986), with the resulting sugar peroxyl radicals then leading to strand break formation at a rate equivalent to that for loss of the initial, fast LSI components. These latter processes are attributed to strand breaks arising from the direct interaction of .OH with the polynucleotide sugar phosphate backbone.

ESR spectra of radicals of single-stranded and double-stranded DNA in aqueous solution. Implications for .OH-induced strand breakage

In situ photolysis at 20 degrees C (argon plasma light source, lambda approximately greater than 200 mm) of oxygen-free solutions containing 2 mM H2O2 and heat-denatured, single-stranded (ss)DNA from calf-thymus resulted in the ESR spectra of the 6-hydroxy-5,6-dihydro-thymin-5-yl (1) and 5-methyleneuracil (3) radicals linked to the sugar-phosphate backbone. They were generated by reaction of OH radicals with DNA. By comparison of the decay characteristics of the ESR signals with rate constants from pulse-conductivity measurements [E. Bothe, G.A. Qureshi and D. Schulte-Frohlinde, Z. Naturforsch., 38c, 1030, (1983)] the thymine-derived radicals (1) and (3) can be excluded as precursors of the fast, dominating component of strand breakage of ssDNA. In the absence of H2O2 from native, double-stranded (ds)DNA an ESR signal was obtained (singlet, g approximately 2.004, delta v1/2 approximately 0.8 mT) which was assigned to the deprotonated guanine radical cation, [G.(-H)] of a DNA subunit. It is assumed that by the UV irradiation the guanine radical cation, (G+.), is generated, either by monophotonic photoionization or by electron transfer to pyrimidine bases. By rapid transfer of the bridging proton from (G+.) to the hydrogen bonded cytosine [G.(-H)] is formed. When photolysis of dsDNA was carried out in the presence of H2O2, reaction of photolytically generated .OH resulted in peroxyl radicals and purine radicals. The oxygen for formation of the peroxyl radicals is probably produced by reaction of [G.(-H)] with H2O2. Photolysis of N2O-saturated solutions containing dsDNA or ssDNA provided another possibility of generation of OH radicals. Under those conditions the .OH-induced radicals (1) and (3) were obtained not only from ssDNA but also from dsDNA.

Photosensitized reactions of poly(U) with tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate

The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio.

E.s.r. studies on the mechanism of hydroxyl radical-induced strand breakage of polyuridylic acid

Reaction of photolytically produced .OH radicals with polyuridylic acid [poly(U)] in neutral solutions resulted in the electron spin resonance (e.s.r.) spectrum of the C(5)-OH-6-yl (= 6-yl) radical 1b of the nucleobase. At pH less than or equal to 4 the spectrum of the base radical had disappeared and instead the cyclic 2'-oxo-3'-yl sugar radical 2a was observed. The assignment of the sugar radical was supported by model reactions with SO4-. as the radical inducing agent. Time-resolved e.s.r. measurements showed that the rate of decay of the 6-yl base radical at neutral pH is virtually the same as that of the strand break (sb) formation. These results prove that: (i) in agreement with an earlier proposal the C(2') mechanism contributes to sb formation of poly(U), and (ii) the decay of the 6-yl radical is the rate-determining step in the reaction sequence leading to strand breakage. The change in the e.s.r. spectra at pH 4 is due to an increase in the rate of sb formation with increasing proton concentration. This effect is explained by generation of the radical cation, 4, from the 6-yl radical and/or by rearrangement of the 6-yl radical into the C(6)-OH-5-yl (= 5-yl) radical in proton-induced reactions and subsequent rapid H abstraction from the sugar moieties by species 4 and/or 5.

Hydroxyl radical-induced strand break formation in single-stranded polynucleotides and single-stranded DNA in aqueous solution as measured by light scattering and by conductivity

Combining conductivity measurements and molecular weight determination by means of low-angle laser light scattering, we have found for the polyribonucleotides (polyuridylic acid (poly(U], polyadenylic acid (poly(A], polycytidylic acid (poly(C] and polyguanylic acid (poly(G] and for single-stranded DNA (ssDNA) that, on average, 8.5 counterions per single-strand break (ssb) are liberated under salt-free conditions. This relationship allows us to estimate, from conductivity measurements alone, G-values of single-strand break formation (G(ssb] for the polydeoxyribonucleotides (polydeoxyriboadenylic acid (poly(dA], polydeoxyribocytidylic acid (poly(dC], polydeoxyribothymidylic acid (poly(dT], polydeoxyribouridylic acid (poly(dU] and polydeoxyriboguanylic acid (poly(dG]. The following G(ssb) values (units of mumol J-1) have been obtained for anoxic conditions: poly(dA), 0.23; poly(dC), 0.14; poly(dT), 0.06; poly(dU), 0.046 and poly(dG), 0.009. Time-resolved conductivity measurements in pulse radiolysis enable us to measure the rate of strand break formation. The rate has been found to be similar for poly(dA) and ssDNA over a range of pH values. Poly(dC) and poly(dU) exhibit conductivity increase components with half-lives similar to those of poly(dA) and ssDNA at corresponding pH values. The implications of these results are discussed.

Gamma-radiolysis of poly(A) in aqueous solution: efficiency of strand break formation by primary water radicals

gamma-Radiation-induced single-strand break formation (ssb) in polyadenylic acid (poly(A] has been determined in Ar and N2O-saturated aqueous solution in the presence of different concentrations of t-butanol. Strand breaks were monitored by a low-angle laser light-scattering technique. The efficiencies for strand breakage caused by solvated electrons, hydrogen atoms and OH radicals have been found to be 0.25, 0.20 and 7.8 per cent, respectively. The efficiency of OH radicals depends only slightly on pH (pH 5.0, 7.5 and 9.0) and is independent of the presence of salt (0.01 mol dm-3 NaC1O4) and of the irradiation temperature (20 degrees C and 70 degrees C). The efficiency of OH for ssb formation obtained in this work with poly(A) is much smaller than that of poly(dA). This is explained by the different molecular conformations of the sugar moiety of poly(A) (3'-endo) and poly(dA) (2'-endo). With increasing t-butanol concentration more strand breaks are formed than expected from simple homogeneous competition kinetics of poly(A) and t-butanol for OH radicals. This result is considered to be due to nonhomogeneous reaction kinetics in the above-mentioned competition. The rate constants for the reaction of OH and H with poly(A) have been determined.

Hydroxyl radical-induced strand break formation of poly(U) in anoxic solution. Effect of dithiothreitol and tetranitromethane

The role of dithiothreitol (DTT) and tetranitromethane (TNM) on the yields of radiation-induced strand break formation in polyuridylic acid (poly(U] was studied in anoxic aqueous solutions at neutral pH by low-angle laser light-scattering. From G (single-strand breaks) as a function of DTT concentration it follows that two different processes lead to OH radical-induced single-strand break (ssb) formation. Only one of the two processes, which accounts for 80 per cent of the ssb formation, is inhibited by DTT, the other one, 20 per cent, is not inhibited. The 'repair' process is attributed to H-donation to the C-6-yl radical of the uracil moiety. The C-6-yl radical is produced by OH addition to the C-5 position of the uracil moiety. It follows that the sugar radicals, in contrast to earlier suggestions, do not seem to be repaired by DTT at the low concentrations used. The strand break formation not inhibited by DTT is induced by radicals other than the uracil-6-yl radical, e.g. the uracil-5-yl or the OH radicals reacting with the sugar moiety. The strong reduction of G(ssb) from 2.3 to 0.2 on addition of TNM is also discussed.

The reverse of the 'repair' reaction of thiols: H-abstraction at carbon by thiyl radicals

Thiyl radicals (RS) formed by the reaction of radiolytically generated OH radicals with thiols, e.g. 1,4-dithiothreitol (DTT), react with cis- and trans-2,5-dimethyltetrahydrofuran by abstracting an H atom in the alpha-position to the ether function (k approximately equal to 5 X 10(3) dm3 mol-1 s-1). The so-formed planar ether radical is 'repaired' by the thiol (k = 6 X 10(8) dm3 mol-1 s-1) thereby regenerating a cis- or trans-2,5-dimethyltetrahydrofuran molecule. In this reaction a thiyl radical is reproduced. Thus trans-2,5-Me2THF from cis-2,5-Me2THF and vice versa are formed in a chain reaction: at a dose rate of 2.8 X 10(-3) Gys-1 and a trans-2,5-Me2THF concentration of 1 X 10(-2) mol dm-3 using DTT as the thiol, G(cis-2,5-Me2THF) = 160 has been found. The chain reaction is very sensitive to impurities and also to disulphides such as those radiolytically formed. 2,5-Me2THF can be regarded as a model for the sugar moiety of DNA where the C(4')-radical is known to lead to DNA strand breakage. The possible role of cellular thiols in the repair of the C(4') DNA radical, and also the conceivable role of thiyl radicals inducing DNA strand breakage, are discussed.

Radiation protection of E. coli strains by cysteamine in the presence of oxygen

The survival of various E. coli K12 strains with defects in the rec system have been measured after gamma-irradiation in air in the presence (0.1 mol dm-3) or in the absence of cysteamine. The results confirm those of Bresler et al. (1978) indicating that the protection by cysteamine in the presence of oxygen is due to an influence on enzymatic repair. The low protection by cysteamine of wild-type cells pretreated with chloramphenicol which prevents protein synthesis, supports the above conclusion. The reason for the absence of a protective effect by OH radical scavenging and H-atom donation is discussed. It is proposed that DNA peroxyl radicals are formed during irradiation in the presence of oxygen and that they are transformed into hydroperoxides by H-atom donation from the intracellular glutathione and the added cysteamine. These hydroperoxides are still dangerous for the cell as indicated by the protective action of glutathione peroxidase observed by Marklund et al. (1984).

The oxygen effect in radiation inactivation of DNA and enzymes

A survey is made of literature data dealing with the influence of oxygen on radiation effects in biologically active DNA and enzymes irradiated extracellularly. There is evidence that oxygen takes part in physico-chemical events, directly or indirectly produced by radiation in several ways: from scavenging reducing primary water radicals to reacting directly with macromolecular radical sites. There is evidence that radiation-induced secondary radicals, originating from a variety of low molecular weight biomolecules, can react with DNA and enzymes in their native state, and produce inactivation. By reaction with oxygen secondary radicals become peroxidized and in this form are generally more harmful to biological macromolecules. There are indications that thiol peroxy radicals can also act in the same way. Possible implications for the oxygen effect observed in vivo are discussed.

Lifetime of peroxyl radicals of poly(U), poly(A) and single-and double-stranded DNA and the rate of their reaction with thiols

Peroxyl radicals of poly(U), poly(A), and single- and double-stranded DNA have been produced by photolysing H2O2 in oxygenated aqueous solution in presence of the substrates. The peroxyl radicals are formed by the reaction of OH radicals with the polynucleotides followed by addition of oxygen. The lifetime of the peroxyl radicals and the rate constant of their reactions with the thiols cysteamine, glutathione and dithiothreithol have been measured by time-resolved e.s.r. spectroscopy. The unusually long lifetimes range from 0.2 to 3.3 s. The activation energy for the decay for all four substrates is 10.3 +/- 1 kcal/mol (43 kJ mol-1). The reaction rate constants with the thiols range from k = 0.8 X 10(4) to 1.3 X 10(5) dm3 mol-1 s-1. The reactions of the thiols with the peroxyl radical of poly(U) are known to prevent strand break formation. This shows that the peroxyl radicals of poly(U) observed by e.s.r. are intermediates in the pathway leading to strand break formation.

Reactions of OH radicals with poly(U) in deoxygenated solutions: sites of OH radical attack and the kinetics of base release

Pulse radiolysis of N2O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2.9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t1/2 less than 4 min at 0 degree C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50 degrees C the half lives are 83 min and 26 h, respectively (Ea (fast) = 68 kJ mol-1, Ea (slow) = 89 kJ mol-1, A (fast) = 1.5 X 10(7) s-1, A (slow) = 1.9 X 10(9) s-1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.

Radiolysis of poly(U) in oxygenated solution

Aqueous N2O/O2-saturated solutions of poly(U) were irradiated at 0 degrees C and the release of unaltered uracil determined. Immediately after irradiation G(uracil release) was 1.5 which increased to a value of 5.3 +/- 0.3 upon heating to 95 degrees C. Thereby all of the organic hydroperoxides (G = 6.8 +/- 0.7) and some of the hydrogen peroxide (G = 1.7 +/- 0.2) was destroyed leaving G(peroxidic material; mainly hydrogen peroxide) = 1.0 +/- 0.7. G(chromophore loss) = 8-11 was measured immediately after irradiation, but no increase was observed upon heating. Addition of iodide destroyed the hydroperoxides and caused immediate base release to rise to G = 4 and further heating brought the value to that observed in the absence of iodide. In contrast, on reducing the hydroperoxides with NaBH4, immediate uracil release rose to only G = 2.8 and no further increase was observed on heating. A major product (G = 2.7) is carbon dioxide. There are also osazone-forming compounds produced (G = 2.7), all of which are originally bound to poly(U). Heating in acid solutions, as is required for this test, releases glycoladehyde-derived osazone (G = 0.8) and further unidentified low molecular weight material (G = 0.9). It is concluded that the primary radicals which cause these lesions are the base OH adduct radicals. In the presence of oxygen these are converted into the corresponding peroxyl radicals which abstract an H atom from the sugar moiety. In the course of this reaction base-hydroperoxides are formed. However, such base hydroperoxides cannot be the only organic hydroperoxides, but some (G congruent to 2.5) sugar-hydroperoxides must be formed as indicated by the increase in base release by the addition of iodide. It is speculated that a sugar-hydroperoxide located at C(3') is reduced by iodide to a carbonyl function at C(3'), a lesion that releases the base, while reduction with NaBH4 reduces it to an alcohol function at C(3') thus preventing base release.